Electronic substrates have hitherto been used for arrangement of functional parts or formation of wiring circuits. In recent years, for miniaturization, high functionalization and integration of electronic devices or parts, there have been increased uses for forming through holes (vias) in an insulating substrate and providing a conductive material in the through holes, thereby electrically conducting both surfaces of the substrate. As methods for electrically conducting both surfaces of the substrate, Patent Document 1 discloses a method of forming an Au plating layer as a metal layer on inner wall parts of through holes of an insulating substrate, and Patent Document 2 discloses a method of completely filling approximately drum-shaped through holes formed in predetermined positions of an insulating substrate with a plating metal.
However, these methods require a plating process having large environmental load, and therefore, are complicated in process and also have low economical efficiency.
There is also known a method, as illustrated in FIG. 1, of filling through holes with a conductor paste (conductive paste) composed of a metal powder and a curable resin, followed by curing to obtain filled vias. However, electrical conductivity is low, because the resin is contained in a conductive material, and heat resistance of a substrate is also low, because of restriction due to the heat resistance of the resin.
Furthermore, there is also known a method of filling through holes with a conductor paste composed of a metal powder, an inorganic binder and a resin, and sintering the metal powder by heating at a sintering temperature of the metal or higher to obtain conductive filled vias. This method is excellent in simplicity, and also provides high electrical conductivity, thermal conductivity and heat resistance, because the resin component is evaporated and decomposed by firing. However, in a via-filled substrate obtained by firing after filling the vias with the conductor paste, gaps or voids are sometimes present between the filled conductor (conductive via part) and a wall surface of the hole part. It can be presumed that a cause for occurrence of these gaps or voids is partially because of contraction of the conductor paste filled in the hole due to solvent removal (drying) or due to sintering the metal powder during high-temperature firing.
When the gaps are present between the filled conductor and the wall surface, at least the following three problems may be caused.
(1) When a conductive film (such as an electrode or wiring) is formed on the substrate surface across the filling part, the conductive film may be cut by the presence of the gaps to cause deterioration of conductive performance or disconnection.
(2) The gaps present on the wall surface may be connected to one another along the wall surface to make it impossible to secure impermeability such as airtightness or solder barrier properties of the filling part (for example, uses requiring the airtightness or solder barrier properties).
(3) When the via-filled substrate passes through a wet process such as welding, as a post process, a chemical or the like may intrude into the gaps to cause failures such as a burst of the via part, swelling of a surface film and discoloration.
In the method using the conductor paste, conductor pastes which can suppress sintering contraction during firing have been proposed. Patent Document 3 discloses a Cu conductor paste comprising a Cu powder, a glass powder and an organic vehicle, and having a volume change rate due to firing of 8% or less, and Patent Document 4 discloses a conductor paste comprising Ag as a main component, conductive metal particles containing a metal powder and metal nanoparticles, a glass and/or an inorganic oxide, and an organic vehicle, and having a volume change rate after firing at 600° C. of ranging from −5 to +10%. When these conductor pastes are used, surface smoothness can be improved, and further, adhesion force between the filling part and the substrate can also be improved.
However, even when these conductor pastes are used, it is difficult to completely eliminate the occurrence of gaps or voids. That is, the above-mentioned problems (1) to (3) cannot be completely solved. The cause for the occurrence of these gaps or voids is because a high conductive metal such as Cu or Ag is low in wettability with a ceramic substrate, by its nature, and cannot wet the ceramic substrate (hole part wall surface) under firing conditions, and therefore, even when the conductor paste having small sintering contraction during firing is used, it is difficult to form a continuous bonding interface layer between the sintered metal and the substrate wall surface. In other words, gaps or voids are liable to occur between the conductor (conductive via part) filled after sintering and the hole part wall surface. In each of the conductor pastes disclosed in Patent Document 3 and Patent Document 4, the glass component blended in the paste is present in the interface between the filled conductor (conductive via part) and the hole part wall surface by melt flow during firing and forms a bonding layer, thereby exhibiting the adhesion force and filling a part of gaps or voids. However, it is difficult to fill up all of gaps or voids, because the ratio of the glass component is limited. Such gaps or voids present along the wall surface are easily connected to one another, leading to impairing the airtightness or non-permeability (a characteristic of preventing a liquid from intruding).
In each of these conductor pastes, the glass component is essential. Therefore, denseness, electrical conductivity and heat radiation cannot be highly improved.
Furthermore, Patent Document 5 discloses a method of forming an oxide layer of an active metal on an inside wall surface of a via, and further forming a conductor layer composed of the above-mentioned active metal inside this oxide layer, thereby improving adhesive force and adhesion force between the inside via conductor and the via wall surface.
However, even when this method is used, it is difficult to highly suppress the occurrence of gaps between the conductive via part and the hole part wall surface. That is, when the sintering contraction of the conductor paste for forming the conductive via part is large, contraction stress exceeds the adhesion force between the conductive via part (conductive via part including the conductor layer) and the via wall surface, and the conductive via part (conductive via part including the conductor layer) is peeled from the via wall surface to form gaps. As a result, the airtightness or the non-permeability of the via-filled substrate is impaired. Patent Document 5 does not disclose the details of the conductor paste for forming the conductive via part, and the airtightness or the non-permeability of the via-filled substrate is not verified.